I. Field of the Invention
The present invention relates to wireless communications. More particularly, the present invention relates to a novel and improved method and system for reducing the bandwidth requirements of satellite or wireless communication networks involving multiple channels.
II. Description of the Related Art
Wireless networks in which multiple communication terminals communicate via radio frequency (RF) signals are well known. In some instances, the RF signals are relayed from an orbiting satellite. In other instances, the RF signals are transmitted to terrestrial based systems. An example of a satellite based system is shown in FIG. 1, but the process and operation of the present invention applies to other communication networks.
Communication using the satellite network of FIG. 1 involves transmission of digital information and control information via RF signals 6 relayed by satellite 8 to and from terminals 10 and master station 12. The invention provides a bandwidth saving method which specifically applies to the transmissions in the direction from the terminals to the master station, referred to the "return link," indicated in FIG. 1.
System like that shown in FIG. 1 generally use multiple frequency-division channels for transmissions to and from the terminals over an allocated RF bandwidth. FIG. 2 shows a set of frequency divided channels, each requiring a minimum amount of RF bandwidth indicated as "F.sub.c " Hertz. The value of F.sub.c is dependent on several factors, including the transmitted symbol or data rate, modulation type, and necessary bandwidth rolloff margin. The resulting necessary bandwidth per channel is referred to as the "occupied bandwidth".
In an ideal system, the occupied bandwidth and the channel separation values could be the same. However, this would require that the frequency source in the terminal (herein referred to as the "frequency synthesizer") provide a transmission frequency reference of the precise value required for the intended channel frequency as measured at the receiving station.
In actual systems, the terminal frequency synthesizer will have frequency errors which vary from terminal to terminal and over time. These frequency errors may be significant compared to the required occupied bandwidth. Additionally, other aspects of the communication path may cause additional frequency errors from the ideal intended frequency of operation. Therefore, the channel separation frequency must be increased to allow for the necessary occupied bandwidth and the transmit frequency uncertainty.
FIG. 3 illustrates the minimum channel spacing given a frequency error range F.sub.e, including the errors by the terminal operation added to the errors of the communication path effects. As shown, in order to ensure that two adjacent channels do not interfere with one another, the channel separation required is F.sub.c +(2.multidot.F.sub.e). The extra bandwidth required by this potential frequency error can add significant cost to the network operation because it reduces the number of channels that can be supported in the given bandwidth. Given the limited nature of RF bandwidth, increased bandwidth requirements increase the cost of communication. Thus, a method and apparatus for reducing the necessary bandwidth requirements would be highly desirable.